Aluminum and zinc complexes supported by functionalized phenolate ligands: Synthesis, characterization and catalysis in the ring-opening polymerization of ε-caprolactone and rac-lactide

A series of aluminum and zinc complexes supported by functionalized phenolate ligands were synthesized and characterized. Reaction of 2-(3,5-R2C3N2)C6H4NH2 (R = Me, Ph) with salicylaldehyde or 3,5-di-tert-butylsalicylaldehyde afforded 2-((2-(1H-pyrazol-1-yl)phenylimino)methyl)phenol derivatives 2a–2d. Treatment of 2a–2d with an equiv. of AlR23 (R2 = Me, Et) gave corresponding aluminum aryloxides 3a–3e, while reaction with an equiv. of ZnEt2 afforded zinc aryloxides 4a–4d. Treatment of 2c with 0.5 equiv. of ZnEt2 formed diphenolato zinc complex 5. All new compounds were characterized by 1H and 13C NMR spectroscopy and elemental analyses. The structures of complexes 3a, 4a and 5 were further characterized by single crystal X-ray diffraction techniques. The catalytic activity of complexes 3–5 toward the ring-opening polymerization of ε-caprolactone was studied. The zinc complexes (4a–4d) exhibited higher catalytic activity than the aluminum complexes (3a–3e). The diphenolato zinc complex 5 showed lower catalytic activity than the ethylzinc complexes 4a–4d. The aluminum complex (3b) is inactive to initiate the ROP of rac-lactide, while the zinc complex (4d) is active initiator for the ROP of rac-lactide, giving atactic polylactide.

Graphical abstractAluminum and zinc complexes supported by functionalized phenolate ligands were synthesized and characterized, and the catalysis of these complexes toward the ROP of cyclic esters was investigated.Figure optionsDownload full-size imageDownload as PowerPoint slide